Mouse intracisternal A particles (IAPs) are defective retroviruses encoded by members of a large family of homologous but non- identical endogenous proviral elements. IAPs are seen in preimplantation mouse embryos, some normal tissues (notably the thymus) and in many mouse tumor cells. Transpositions of IAP el- ements have caused insertional mutations at both the germ line and somatic cell levels. It is not known whether, or how, particular IAP elements are selectively activated in a given cell type. We are examining the regulation of IAP expression in two ways: (1) through biochemical analysis of site-specific nuclear protein interactions with a cloned IAP LTR known to be promoter competent in vivo; and (2) molecular genetic analysis of IAP activation in thymus cells of inbred mouse strains that differ greatly in their levels of IAP gene expression. The first approach has defined five protein-binding sites in the LTR region upstream of the CAAT and TATA boxes, and revealed that nuclear extracts from different cell lines vary widely in the absolute and relative amounts of binding proteins. Factor concentrations were higher in cells transformed by nuclear oncogenes than those that were not, and correlated well with the relative promoter activity of the LTR when transfected into the corresponding intact cells. IAP expression is known to be sensitive to CpG methylation in the LTR. We have now shown that methylation affects protein binding to one of the five identified sites. In the thymus study, we are determining the number of different IAP transcripts in cells of high and low producer strains by partial sequencing of cDNA clones prepared with IAP-specific primers, and developing methods for the cloning of the corresponding transcriptionally active genomic elements. We are also examining the mechanism which IAP expression is suppressed in Fl progeny of crosses between high and low IAP producer mouse strains. Collaborative studies are also continuing on the possible role of IAP protein expression in the pathogenesis of a genetically determined mouse diabetes.